Rational Selection of Cyanines to Generate Conjugate Acid and Free Radicals for Photopolymerization upon Exposure at 860 nm

Abstract Different cyanines absorbing in the NIR between 750 and 930 nm were applied to study the efficiency of both radical and cationic polymerization in combination with diaryliodonium salt. Variation of the connecting methine chain and structure of the terminal indolium moiety provided a deeper insight in the structure of the cyanine NIR‐sensitizer and the efficiency to generate initiating radicals and conjugate acid. Photophysical studies were pursued by fluorescence spectroscopy providing a deeper understanding regarding the lifetime of the excited state and contribution of nonradiative deactivation resulting in generation of additional heat in the polymerization process. Furthermore, electrochemical experiments demonstrated connection to oxidation and reduction capability as influenced by the structural pattern of the sensitizer. LC–MS measurements provided a deeper pattern about the photoproducts formed. A nonamethine‐based cyanine showed the best performance regarding bleaching in combination with an iodonium salt at 860 nm.

In recent years,f undamental progress has been made in this field particularly to dry coatings. [14-18, 20, 22, 23, 32] This can physically occur to remove water as solvent resulting in formation of as olidified film. [14,32] In addition, chemical drying results in crosslinked films applying either liquid monomers [15-18, 20, 22, 23] or reactive powders. [15,17,19,21] Application of the latter belongs to green technologies as well. Consequently,NIR-lasers with line-shaped focus [18] or recently introduced high-power NIR-LED devices [16,32,37,38] have moved into the focus of the aforementioned applications.I n addition, chemical drying based on activated photoinduced electron transfer (PET)m ostly results in colored products, [5,22,37,38] which can be beneficial to design materials with readout function at certain wavelengths.
Alternatively,u p-conversion nanoparticles have received additional attention to initiate photopolymerization either in the UV or blue region. [18,[39][40][41][42][43][44][45][46] Here,a bsorption of the laser light proceeds at 980 nm while at hree-and four-photon absorption results in formation of blue and UV light. [40,41] This was applied to initiate either free-radical [40-42, 44, 45, 47] or controlled radical [41] polymerization. In addition, these systems can generate conjugate acid [39] to initiate cationic polymerization. [43] Additional interest relating to applications focused on establishing deep cure length (more than 13 cm), [44] and 3D printing [47] that represents some interesting features of these materials.Such NIR responsive materials generate only circa 1% up-converted radiation [48] that is used in photonic events while the remaining part belongs to thermal energy released in the surrounding matrix.
Furthermore,there exists an approach combining physical and chemical drying of aqueous dispersions comprising crosslinkable monomers for radical polymerization. [14] Here, ac ombination of physical NIR-drying applying high-power NIR-LEDs emitting either at 820 nm, 860 nm, or 930 nm facilitated film formation of the aqueous poly(urethane) dispersion while subsequent exposure with aU V-LED emitting at 395 nm resulted in formation of as emi-interpenetrating polymer network by radical crosslinking of the multi-functional acrylate monomer. [14] Ethyl(2,4,6-trimethylbenzoyl) phenylphosphinate generated radicals to initiate free-radical polymerization of multi-functional monomers such as tripropylene glycol diacrylate (TPGDA). This procedure resulted in films appearing colorless by eye,which can be seen as abig effort in this field. [18] Herein, cyaninesalso served as absorber for physical drying. [14,32] Big efforts have been made in both radical [5,16,37,38,49] and cationic [5,16,37,49,50] polymerization using NIR-LEDs.P hotochemical generation of initiating radicals and conjugate acid based on activated PET explain the use of strong emissive sources such as NIR lasers [18,40,41,44,47] or more sophisticated high-power NIR-LEDs. [5,16,37,38,49,50] Heptamethines comprising either an indolium or benzoindolium moiety fit well in this scheme. [5] Particular cyanines with ad imethylene bridge in the center of the cyanine showed promising performance regarding sensitization of cationic polymerization in the NIR. [16,37] However,t hose with trimethylene bridge failed to cure monomers where polymer formation proceeded by ac arbo cation. This was an epoxy monomer. [20] There are still open questions regarding the optimal pattern of the cyanine sensitizer in activated PET reacting with an initiator such as an iodonium salt. [22,51] Thesensitizer can be ac onjugated system with either open or bridged polymethine chain. Thepattern of the indolium moiety differs or the length of the polymethine chain can differ.E ach extension of the polymethine by two methine groups typically results in abathochromic absorption shift of 100 nm. [1-5, 7, 52-55] In addition, comparison of acyanine comprising an indolium moiety with that of ab enzo[cd]indolium end group shows a2 00 nm bathochromic shift of absorption. [5] Nevertheless,only some of these cyanines worked well to sensitize both radical and cationic photopolymerization. There still exists al ack regarding the choice of the general cyanine pattern relating to photochemistry and photophysics in such systems.S tructural features such as polymethine length, substitution of polymethine chain with bridging moiety,s ubstitution of the meso-position, and type of indolium pattern have become some targets of research. Nevertheless,f ocus shall be given on those sensitizers facilitating the use of high-power NIR-LEDs emitting between 800 and 950 nm. Nowadays,semiconductors have been available for acceptable economic conditions emitting at 860 nm or 930 nm. Thus,t his contribution is going to bring more impetus in this field regarding the optimal structural pattern of the NIR sensitizer for this purpose and their efficiency to initiate radical and cationic photopolymerization.
Substitution of the 4-position with respect to the indolium nitrogen in 1aand 2aby an alkoxy group results in 1band 2b, respectively.T his leads to 60 nm bathochromic absorption shift. Change of the benzo[cd]indolium moiety in 2a by indolium and increase by additional two methine groups in the connecting polymethine chain results in 3a exhibiting ah ypsochromic absorption shift of about 100 nm. 3a assigns to ah eptamethine while additional extension of the polymethine results in the nonamethine 4 exhibiting abathochromic shift of about 100 nm.
Thea bsorption of the heptamethine pattern of 3b-3d overlaps with the emission of the 860 nm NIR-LED either at the absorption tail of Sens (3b, 3d)o rt he absorption maximum of Sens (3c). Ad imethylene bridge in the center of the conjugated system keeps the geometry nearly planar. [38] Similar structural patterns of 3b and 3d contributed to initiation of radical and cationic polymerization [5,16,49] while heptamethine-based cyanineswith amoiety comprising atrimethylene bridge in the center failed regarding initiation of cationic polymerization. [20] Formation of nucleophilic photoproducts inhibits cationic polymerization. In addition, 4 assigns to an onamethine cyanine in which the substituted trimethylene bridge slightly distorts the planarity of the conjugated chain. 3a comprising no bridging elements at the polymethine showed an almost planar geometry in quantum chemical calculations. Figures SI7-13 depict the geometries of 1-4.B ridging of the central position with two methylene groups results in 3b and 3c exhibiting almost aflat pattern of the NIR-absorber.A bsorber 4 also exhibited planarity.
Electrochemical experiments complement the properties of the conjugated structures.T he oxidation potential (E ox ) drops by increase of the polymethine chain as concluded by comparing 1a and 1b as well as 2a and 2b.Asexpected, the introduction of an additional electron-donating substituent results in an easier oxidation as shown by 1b and 2b. Interestingly, 2b, 3,a nd 4 possess similar ability to oxidize although they exhibit distinct substitution patterns and number of p-electrons.Thus,itdoes not really matter whether the connecting conjugated chain was open as in 2a/3a or bridged as in 3b-d.T he latter comprises either an amino or phenyl substituent in the meso-position.
However,t he situation changes when comparing the reduction potentials (E red )of1-4.Here, 1a, 2a,and 4 exhibit similar data although they possess different structural patterns. E red of 3 appears at about À0.6 Vd emonstrating less efficient reduction capability compared the compounds,s ee above.N evertheless,t he systems disclosed report about generation of conjugate acid and initiating radicals generated according to an oxidative mechanism, where E ox , E red of the initiator and the excitation energy (E 00 )o ft he sensitizer determine whether the reaction can proceed from at hermodynamic point of view.Thus,the free reaction enthalpy (DG el ) can be written as DG el = F(E ox ÀE red ) < M-> E 00 (F = Faraday constant). [58,59] According to these data, DG el appears around À0.2 eV in the case of 1b.1a , 2,a nd 3c show DG el around À0.3 eV while that of 3b and 3d appears at À0.4 eV. 3a and 4 exhibited the lowest DG el -values with À0.5 eV and À0.7 eV, respectively.T hese changes can be seen more or less as Table 1: Comparison of the photophysical data such as absorption maximum l max ,extinction coefficient e max ,fluorescence emissionmaximum l f max , fluorescence quantum yield F f ,fluorescence decay time t f ,a nd rate constant for fluorescence k f for the cyanines 1-4 and their respective oxidation products 3b-ox.Electrochemical data relate to their oxidation E ox and reduction potentials E red .I naddition, data also show the concentrationo f conjugatea cid ([a H þ ]) formed after exposure of the system comprising the sensitizer and 5 after 15 min (see ref. [20] for more details regarding quantification of conjugateacid formed using RhodamineBlactone, SI shows more details).The quantity k b (rel) discloses the change of sensitizer concentration after 2min NIR exposure at either 820 nm (1a, 3a, 3b, 3d)or860 nm (1b, 2, 3c, 4)with high-intensity NIR-LED device emitting with an intensity of 1Wcm À2 .The quantity was corrected by the absorption of the sensitizer; that is, dividing of D[Sens]/Dt by (1-10 ÀOD[Sens] ). moderate comparing those of 1 and 2 as well as data of 3b-d.
This generally indicates that PET should occur from at hermodynamic point of view.The cation of 5 served as oxidizing substrate exhibiting E red of À0.69 V. [10] Solvent effects,i nternal reaction coordinates, [60] and internal activation barriers [16,38,60] are not included here.N evertheless,r esults obtained demonstrate huge differences regarding the reactivity.

Properties of the First Excited Singlet State
Stationary and time-resolved fluorescence measurements explain the emission characteristics of the excited state to receive adeeper understanding of the S 1 considering the NIRsensitizers shown in Scheme 1. Here, 1a, 1b,a nd 2a exhibit both low fluorescence quantum yield and decay time.Some of these data (1a and 2a)r eside in the range of ap revious report. [61] Non-radiative processes mainly contribute to the overall behavior of the excited state,T able 1. Size increase of the conjugated pattern decreases excitation energy resulting in an increase of internal conversion (IC). Obviously,t here exists in these structures ah igher probability where av ibration with higher energy of the S 0 may couple with the lowest vibration of the S 1 favoring IC. [62] Those non-radiative events therefore proceed more efficient in the case of absorbers comprising ab enzo[cd]indolium pattern such as 1 and 2 as shown by the low t f and F f data, Table 1. This follows by comparison with 3 and 4 exhibiting higher values,a nd of course higher reactivity with 5 as documented by the bleaching constant k b and the amount on conjugate acid formed with respect to sensitizer concentration ([a H þ ]/[Sens]), Table 1. In general, non-radiative events can lead to temperature increase in adiabatic systems,w hich promotes overcoming internal activation barriers in PET existing in similar systems. [6,16,38] In addition, the fluorescence decay time of 2a was lower as the time resolution of the single photon equipment used to study dynamics of the excited state;t hat is, > 25 ps.T ime-resolved data were not available in the case of 2b because spectral response of our instrument ends above 910 nm.
In addition, increase of the conjugated system from 1a to 2a by insertion of an additional CH = CH group as expected results in ad ecrease of the energy gap between the ground state and the excited state.T his therefore leads to ah igher probability of energetically higher available vibrations of the ground state (S 0 ). It favors internal conversion (IC) between the excited singlet state (S 1 )a nd S 0 .H ere,o nly totalsymmetric vibrations couple with the symmetry-allowed electronic transition. [63] Excess of vibrational energy is distributed by vibrational relaxation (VR) to the isoenergetic asymmetric and symmetric states related to intramolecular vibrational redistribution (IVR). [64] Thee lectronic spectra of cyanines exhibit av ibrational fine structure where the energy difference between the sub-bands resides at about 1200 cm À1 AE 200 cm À1 . [65,66] The dominant symmetric carbon-carbon valence vibration of the polymethine chain determines spacing between subbands. [65,66] Nevertheless,t he molecule still remains in the S 1 exhibiting al ifetime in the ps and sub-ns frame after VR of the S 1 ,T able 1. Although VR within the S 1 results in release of thermal energy,the contribution to the overall amount of heat released into the surroundings can be seen more or less as minor amount.
IC is believed to be the main source for releasing heat into the surroundings. [5] Here,the energetically lowest vibration of the S 1 (v' = 0) couples with ahigher vibration of the S 0 (v = n) resulting in av ery hot molecule.T his proceeds according to the above-mentioned prerequisites.P ractically,this excess of energy is redistributed by vibrational cooling (VC). [64] It transfers its excessive energy by collision with matrix molecules relating to vibrational cooling. Recent studies showed several examples where the temperature can rise higher than 100 8 8Cand sometimes even more. [14,[16][17][18][19]32,37,38] In the case that no VC would proceed, the extremely hot molecule would burn because it possesses so much energy that it cannot be fully transferred to IVR.
In general, the evolution of heat in systems exhibiting large contribution of IC has not been well understood. Scheme 2d epicts the occurring processes from the simplest point of view keeping in mind that the scenario appears more difficult. [62,64] Experiments pursued in this contribution indicated at emperature increase using TPGDAa sm atrix upon exposure at 860 nm, see Figure SI14. Te mperature evolution observed describes the overall increase as affected by the heat capacity of the film. In general, this temperature can be much higher as,for example,observed during the burning of CD-R and DVD-R where cyanines also took ak ey function. [9] It exceeded for as hort moment av alue above the decomposi-Scheme 2. Schematic representation of the electronic ground state (S 0 )with its vibrationalstates v = 0, v = 2, and v = n,t he first electronic excited state (S 1 )w ith the respective vibrationals tates v' = 0, v' = 1, and v' = 2, and the vibronic (vibrational-electronic) transition where one-photona bsorption (OP) proceeds with the energy hn. This is followed by relaxation from the higher vibrational levels into the lowest vibrational level (v' = 0) of the first excited singlet state by transfer of excess vibrational energy into isoenergetic vibrations by intramolecular vibrational energy distribution (IVR). This finally results in population of the lowest vibrational level of the S 1 (see also ref. [64]). The S 1 reacts either with the cation of 5 resulting in formation of initiating radicals and conjugate acid or it can couple with ah igher available vibration of the S 0 resulting in the release of heat caused by vibrational cooling (VC) as discussed earlier. [64] tion temperature of the absorber as indicated by its bleaching. In general, the lower the excitation energy,the higher the rate of non-radiative deactivation as shown for several emission studies of cyanines. [5,10,20] Equation (1) shows the relation between fluorescence quantum yield F f ,fluorescence decay time t f ,rate constant for fluorescence k f ,non-radiative deactivation k nr ,and areaction based on activated PET k PET .Thus,the larger the contribution of non-radiative events,t he lower become F f and t f ,a nd therefore,also the lower the probability to participate in PET. Since internal conversion proceeds according to unimolecular kinetics,t his reaction favors deactivation of the excited state rather than abimolecular reaction as expressed by k PET [Q] ([Q] = concentration of the substrate reacting with the S 1 ). Fluorescence measurements cannot distinguish whether internal conversion or PET dominates the non-radiative deactivation process.H ere,s tudy of photoproducts as shown below gives deeper insights into the mechanism. Figure 1e xhibits representative fluorescence decays of some sensitizers (1a, 1b, 3a-c,a nd 4). Data obtained give access to k f ,p roviding information how efficient radiative deactivation by fluorescence can proceed. It shows about 10 times higher values in the case of 3a and 3b compared to 1b.O bviously,t he benzo[cd]indolium moiety favors nonradiative deactivation. Theoretical considerations discuss the enhancement of this process by vibrational coupling. [61] Thus, the longer polymethine chain may direct the system to more radiative deactivation while larger stiff conjugated patterns of the terminal group as available in 1 move it toward nonradiative deactivation. Interpretation of data does not go straightforward by comparison with those of 3. 3a and 3b exhibit both similar t f and F f resulting logically in comparable k f although 3a possesses an open polymethine chain. Moreover, 3b comprises abridge with two methylene groups in the center of the polymethine and ap henyl ring in the mesoposition.
Replacement of the phenyl group in 3b by ad iphenyl amino group results in 3d showing similar fluorescence decay of 934 ps,T able 1. F f dropped caused by replacement of Ph by N(Ph) 2 in the meso-position. Consequently, k f decreased. In addition, 3c exhibits al ower decay rate indicating that both benzo[g]indolium and N(Ph) 2 significantly contributed to the fluorescence dynamics.T he k f value of 3c is about ah alf compared to 3d.Moreover,anelectron-withdrawing group as the phenyl ring in 3b did not cause significant changes of both non-radiative and radiative deactivation in comparison with the respective diphenyl amino compound 3d.3 cpossesses al ower emission decay rate.O ne can summarize in brief: t f (3b) = 1008 ps, t f (3c) = 213 ps, t f (3d) = 934 ps.
Interestingly,t he oxidized photoproduct, available after PET of 3b resulting in 3b-ox,s howed similar decay rates although the oxidized dimethylene bridge comprises two additional p-electrons.I ts electronic pattern appears similar to that of fulvenes. [16,38] Thus,t hese structural changes in 3b-  869 nm)). SI provides more experimental details (red:i nstrumental response of ascatter comprising Ludox in water whose detection was close to the excitation wavelength at 670 nm, blue: decay curve of the sample, black:c alculated decay by iterative convolution between the instrumental response function and exponential decay). Time-correlated single photon counting was applied to collect the data. ox do not significantly affect the decay dynamics as concluded with those of the respective cyanine 3b.D ata also demonstrated sufficient reactivity with 5 although there was only as mall overlap between absorption of 3b-ox and 5, Table 1.
Moreover,c yanine 4 exhibiting an onamethine chain between the indolium rings has as horter decay time.I ts k f resides in the range of 3c although it comprises al onger polymethine chain between the benzoindolium rings.T he significant bathochromic-shifted absorption of 4 compared to 3 results in adecrease of the energy gap between ground and excited state therefore resulting in an increase of nonradiative deactivation such as internal IC.
Consideration of DG el by using the oxidation potential of the sensitizers and the reduction potential of 5 (E red = À0.69 V [10] )r esults in slight negative values of DG el .T hus, 1-4 should exhibit similar reactivity since DG el appears similar. However,d ata obtained for bleaching (k b )a nd generation of conjugate acid (a H þ )s hown in Table 1r equire adiscussion of ascenario that requires inclusion of additional points to understand these reactivity differences.T hese data represent the chemical reactivity of the system. Figure 2 shows the spectral changes obtained upon exposure of Sens in the presence of 5 with an 860 nm high-intensity LED.T his includes ab enzo[cd]indolium derivative (2a)w ith open polymethine chain (5 methine groups) in Figure 2a,b enzo-[g]indolium derivative (3c)w ith dimethylene bridged polymethine chain (7 methine groups) in Figure 2b,a nd an indolium derivative (4)w ith trimethylene-bridged polymethine chain (9 methine groups) in Figure 2c.
Data obtained for the decrease of sensitizer absorption in combination with 5 during exposure indicate either as low decrease of OD (Figure 2a), as ignificantly faster decrease and therefore also bleaching at the exposure wavelength (Figure 2b), and finally the fastest decrease of OD with formation of anew absorption band between 400 and 500 nm (Figure 2c). General structures 1 and 2 are related to the small spectral changes shown in Figure 2a while 4 is assigned to Figure 2c. 3c resulted in the pattern shown in Figure 2b where an ew absorption appears hypsochromically shifted at 600 nm. This may relate to the oxidized species exhibiting afulvene pattern as similarly disclosed in the case of 3b-ox. [16]

Research Articles
These different bleaching efficiencies enable to draw the following reactivity ratio;that is,according to Table 1 It should also result in as imilar ratio considering the formation of conjugate acid [a H þ ]. [20] Photoinduced electron transfer between photoexcited Sens (Sens*) and 5 results in the oxidized form Sens + C,E quation (2), which stabilizes by release of conjugate acid [a H þ ]and photoproducts comprising nitrogen (Pr(N)) and those with no nitrogen (Pr(O)), Equation (3). This equation shows that one mole of conjugate acid should relate to one mole sensitizer.P hotoproducts comprising amino groups (Pr(N)) can additionally react with Sens + C that yields back Sens,E quation (4). In addition, protonation of Pr(N) according to Equation (5) also reduces the available amount on conjugate acid needed to protonate colorless Rhodamine Blactone resulting in deep red colored Rhodamine B. This reaction was qualified to probe quantitatively the amount on acid formed. [20] It can additionally explain why its available amount can be smaller than expected.
Sens þ C þ PrðNÞ!Sens þ PrðNÞ þ ð4Þ According to the data shown in Table 1, the following ratio was obtained for the formation of conjugate acid upon exposure of Sens in the presence of 5.
Mass spectrometric analysis obtained by aL C-MS analytical protocol enabled to draw possible pathways based on the molecular ions detected as shown in Scheme 3i nt he case of 4 (for details see SI). Explorative MS studies of exposed solutions comprising 3b and 3d were previously reported. [16,38] Thus, 3c should exhibit similar behavior compared to 3c.T he reaction between Sens*a nd 5 resulted either in oxidation of position iii or bond cleavage at the position i and ii resulting in formation of nucleophilic products.F or simplification, sensitizers exhibiting an open chain such as 1, 2,a nd 3a were not included here since molecular ions observed exhibited amass related to cleavage of the adjacent bond with respect to the indolium moiety. Surprisingly, 4 showed oxidation activity at its iii-position resulting in yellowish photoproducts as shown in Figure 2c while the substrate appeared optically open at the excitation wavelength. Such unexpected behavior may facilitate the design of photoswitching systems.Basically cleavage at either the i or ii position was expected as main pathway since heptamethine derivatives with similar trimethylene bridge in the center showed this behavior with no indication of products based on oxidation of position iii. [22] Scheme 3 summarizes the reaction products observed in case of 4 as ap roposal derived from the molecular ions found in the MS spectra. Thef ormation of this product mixture may also explain why absorption spectra in Figure 2cdo not go through an isosbestic point which typically occurs in case of photochemistry proceeding from A*!B. [67] Structures b 2-4 may explain the hypsochromic-shifted absorption whose pattern does not belong anymore to acyanine.

Activated Free-Radical Polymerization
Recent investigation of cyanines as NIR-sensitizers showed the necessity to introduce additional heat needed for PET with diaryliodonium salt exposed at 805 nm. [16,38] An internal activation barrier requires the introduction of additional heat to succeed with PET in the case of sensitzers comprising cyanine patterns. [6,16,37,38] This can optimally occur under adiabatic conditions with asensitizer exhibiting alarge contribution of non-radiative deactivation, Table 1. Only sensitizers comprising ab arbiturate and therefore no charge required significant lower exposure intensity to initiate successful radical photopolymerization of the monomer TPGDA. [10,20,22,51] In addition, no successful result has been reported regarding the use of ac yanine sensitizer exhibiting an open methine chain between the terminal indolium groups where excitation above 750 nm resulted in generation of aryl radicals and conjugated acid applying 5 as coinitiator. Sensitizers used in preliminary studies comprised aconnecting bridge. [16,18,22] There does not exist ac lear answer how the structural pattern of the cyanine-based sensitizer affects the internal barrier of the PET.T his relates to the length of the polymethine chain and its sensitizing efficiency in PET applying either high or low exposure intensity in the NIR. There exist af ew reports about successful application of cationic cyanines but research did not go into much detail with respect to the structure. [16,38] Thus,c omparison of 1-4 may give some answers showing whether an open chain as in 1, 2,a nd 3a or those comprising an aliphatic bridge as depicted in the case of 3b-3d and 4 affect the sensitization efficiency.Ingeneral, it should follow the reactivity as shown by k b in Table 1a pplying ahigher-intensity NIR-LED.
In addition, the different pattern of the indolium moiety might affect sensitization efficiencyasconcluded by comparison between those comprising benzo[cd]indolium (1, 2), benzo[g]indolium (3c), and indolium pattern (3a, 3b, 3d, 4). In particular, these structures might also affect the size of the internal activation barrier of PET.T he tendencyo fs tronger non-radiative deactivation of 1 and 2 may move them into the focus since ac ombination of sensitizer and 5 also resulted in ad ecomposition temperature below 100 8 8C. [20] According to the reactivity above-mentioned results,s tructures 3 and 4 should lead to higher reactive systems,w hile 1 and 2 would not follow.
Preliminary studies indicated the possibility to overcome internal activation barrier of systems comprising 3b and 3c in combination with the cation of 5 [16,38] and ah igh-intensity NIR-LED.S tructures 1, 2, 3a,a nd 4 appear new in this application field. It was also shown that adimethylene bridge as shown in 3b and 3d favors cationic polymerization, [16,37,38] while sensitizers with trimethylene bridge failed. [20] Surprisingly, 3a exhibited acceptable radical photopolymerization upon exposure with low-intensity emitting NIR-LEDs;that is, 45 mW cm À2 .None of 1, 2a, 3b-c,or4 sensitively responded at this low intensity.E xposure in ap hoto-DSC setup applying higher intensity (l = 860 nm, I = 350 mW cm À2 )g ave similar results.H ere,t he experimental conditions in the photo-DSC facilitate almost isothermal conditions.Thus,heat released by the sensitizer and also by polymerization would not be accessible by the system to overcome internal activation barriers.However,this differs in the case of areal-time FTIR setup where heat formed in the reaction does not efficiently leave the system, rather resulting in conditions that reside between an adiabatic and isothermal system in the time frame of the reaction. This has often facilitated NIR-sensitized radical photopolymerization needing both photons and heat to proceed PET according to an activated scheme. [5,16,37,38] Surprisingly, 3a exhibited an acceptable photopolymerization efficiency using al ow intensity LED (l = 770 nm, I = 45 mW cm À2 ), Figure 3a.O ntheo ther hand, the remaining cationic sensitizers 3c and 4 did not show remarkable polymerization. Temperature increase in the DSC did not change the scenario.O nly 3d slightly responded. [16] Thus, 3a can be seen as an interesting alternative compared to 3b-d in combination with low-intensity NIR-LEDs because it exhibits alower intrinsic activation barrier as concluded by the higher reactivity,F igure 3a,b.O bviously,c yanines with unbridged polymethine chain result in PET with 5 even under low exposure conditions if they bear an indolium moiety as terminal group.A sac onsequence,t he threshold to initiate radical polymerization is lower in 3a.T his might also open new perspectives for applications operating with significantly lower intensity. Mediated Acid-Initiated Cationic Polymerization Self-Polyaddition of 4-(Hydroxybutyl) Vinyl Ether Table 1s hows substantial generation of conjugate acid in the case of 4 and 3c while it results in less quantity in the case of 1, 2,and 3b.Nevertheless, 3a generated the largest amount on conjugate acid. Figure 4shows these reactivity differences, which differently appear in the reactivity of the monomer 4-(hydroxybutyl) vinyl ether (M1). In previous investigations this monomer exhibited fast reactivity [68] with sensitizers comprising indolium terminal groups in combination with an iodonium salt derived from aluminates ([Al(O-t-C 4 F 9 ) 4 ] À ). [37] Figure 4c learly demonstrates the fast reactivity of the bridged derivatives 3c and 4 in combination with 5 whose quantitative amount of conjugate acid remains at significantly higher level compared to 2aand 1b.3balso reacted fast while 3a appeared less reactive compared to the above-mentioned sensitizers of group 3.H ere,b ond cleavage of the polymethine chain results in formation of nucleophilic products inhibiting cationic photopolymerization. Thus,o xidation of position iii favors formation of less nucleophilic products in the case of 3b-d explaining the higher reactivity shown in Figure 4. In this series, 3a possesses ah igher reactivity compared to 1 and 2.T hus,t he introduction of sensitizers exhibiting ab enzo[cd]indolium pattern has not brought progress in this field since these cyanines mostly favor nonradiative deactivation rather than to react with 5 by PET.I t appears more likely that incorporation of al onger methine chain as in either 4 or 3c favors the reactivity in NIRsensitized acidic catalyzed polymer formation of M1.I ta lso shows that these systems worked with 5 comprising an anion that typically failed in cationic photopolymerization;t hat is, [(CF 3 SO 2 ) 2 N] À . [20] Them olecular weight of the isolated polymer indicated a M n of 1053 gmol À1 (M w /M n = 2.6). Such relatively low polymerization degrees (approx. 9) were additionally observed in previous investigations where formation of higher molecular weight materials proceeded in two competitive reaction pathways;that is,traditional cationic polymerization of the vinyl group and apolyaddition reaction favored by the ring closure of the hydroxybutyl ether. [69][70][71] As elf-polyaddition, favored by the structural pattern of M1,may explain the competition of this reaction in acidic environment since the reactivity of this conjugate acid-initiated polymer formation proceeded with acceptable reaction rate comprising [(CF 3 SO 2 ) 2 N] À .O nium salts comprising this anion show less reactivity in systems following rather at raditional cationic polymerization protocol where chain growth proceeds by the carbocation as intermediate.Oxiranes there typically serve as monomers. [20] Here,t he nucleophilicity of the anion appears too high to accomplish ah ighly reactive system in cationic polymerization since the higher nucleophilicity of this anion avoids an efficient chain growth according to ac ationic polymerization mechanism. Obviously,t he self-polyaddition leads to the polymer shown in Scheme 4. [70,71] It can obviously tolerate higher nucleophilicity of participating anions/species. This may have an impact on future developments in this field since poly(4-hydroxy vinyl ether) may contribute as plasticizing agent in systems forming semi-interpenetrating polymer networks based on radical polymerization using multifunctional (meth)acrylic esters and monomer M1.
Afurther point requires attention. The a-hydrogen of the ether carbon possesses al ower C À Hb ond dissociation energy. [72] Thus,a ne lectrophilic radical formed in the initiation mechanism can easily abstract ah ydrogen from there resulting in anucleophilic radical;that is,CH 2 =CHÀOÀ CCH À R. This intermediate should be easily oxidized by reaction with the onium salt resulting in formation of the respective cation CH 2 = CH À O À C + H À Ra sp reviously shown by alternative reactions with onium salts. [73] It is stabilized by release of conjugate acid explaining the high reactivity of vinyl ethers in cationic polymerization. [68] Our experimental results support these findings.T hus,e xposure of 3a and 4 in the presence of 5 resulted in an increase of [a H þ ]/[Sens]to0.41 and 0.33, respectively,exposing in CH 3 CN:M1 = 4:1(vol %). This relates to an increase of 36 %inthe case of 3a and 60 % in the case of 4 with respect to the data received without vinyl ether M1,T able 1. It also evidences the aforementioned hypothesis that nucleophilic radicals can react with 5 resulting in formation of conjugated acid.
In addition, the aforementioned carbocation formed can also competitively add M1 resulting in branched structures (see Figures in SI showing that NMR spectra do not only comprise structural elements of Scheme 4a nd shorter polymerization degree.  . Conversion-timep rofiles of the vinyl ether M1 obtained with the initiator system comprising initiator 5 (2.0 wt %) and NIR sensitizer ([Sens]:5 = 1:6(molar ratio)) exposed at 860 nm (I = 1.0 Wcm À2 ) in the case of 4, 3c, 2,and 1b and 820 nm (I = 1.0 Wcm À2 )i nthe case of 3a, 3b,and 1a measured by real-time FTIR.

Conclusion
Sensitizers 3a and 4 have brought new impetus in this field. Their electronic structure exhibits either an open and non-bridged polymethine chain as in 3a or even al onger polymethine chain as in 4,r espectively.S urprisingly, 3a required al ower activation barrier in photoinitiated radical polymerization compared to derivatives exhibiting the same number of polymethine moieties;t hat is, 3b-d.F uture developments may focus on the design of cyanine-based sensitizers exhibiting ap olymethine chain with no bridging moieties while availability at larger scale may give rise to further issues.
Structures 1 and 2 comprising ab enzo[cd]indolium pattern showed good bathochromic shift of absorption on the one hand side but lower sensitization activity on the other hand. These compounds may become of interest to generate heat on demand just by turning on al ight source in technologies where nowadays oven technologies operate either to initiate chemical reactions based on thermal activation or physical events such as removal of volatile components in coatings.R ecent demands of the society to save energy and resources have enforced us to pursue such strategies.F rom this point of view,t his contribution brings valuable aspects in this field regarding future design of cyanines for their use in photopolymer systems;t hat is,t o connect them to photoinitiate either chemical events where 3 and 4 present acceptable structures or to photoinitiate physical events such as photonic drying with structures based on 1 or 2.
Based on the results obtained with 4,i tw ould be interesting to study if incorporation of additional methine moieties,w hich would typically result in as ignificant bathochromic shift of absorption, will also enable excitation wavelengths around 1000-1100 nm. Tw om ethine moieties typically result in a1 00 nm bathochromic absorption shift. This spectral region still challenges to design cyanines showing sufficient PET at this spectral region. Nowadays, available compounds mainly generate heat upon excitation on demand and therefore function only as absorbers.Hopefully, the design of respective structures will bring additional impetus in the near future.
Theg eneral question will also arise about the practicability to access such structures.From this point of view, 3b-d provide easier access due to the availability of the connecting bridge available by Vilsmeier reaction. Here,n ew directions should come to develop alternative synthetic routes giving access to more structures with open polymethine chain absorbing around 1000 nm, keeping in mind the feasibility to access such structures and to receive materials resulting in acceptable shelf-life in systems comprising iodonium salt and vinyl monomers.